Intravenously administered hemopoietic cells colonize the BM extravascular spaces that support their proliferative expansion/differentiation. This phase of hemopoietic engraftment is preceded by a phase of initial capture of the cells within BM sinusoids followed by adhesion/migration steps leading to their firm anchoring within BM. The molecular pathways responsible for this initial phase are distinct from the ones dictating their engraftment, however, they are poorly understood, and thought to involve members of the integrin family and chemokines. In preliminary data we have shown that the VLA-4/VCAM-1 pathway has a dominant role in the initial phases of homing, but a multi-component participation in adhesion/migration steps is likely. The identity of cooperating molecules and their functional contribution to specific phases in homing remain to be delineated. In this application we will expand our observations using a multi-parameter approach incorporating genetic mouse models with conditional ablation of specific target molecules (VCAM-1) or using mice harboring hemopoietic cells with altered Gi protein signaling. The participation of VCAM-1 expressed by hemopoietic cells - a novel finding - or by hemopoietic stroma/endothelial cells in the initial stages of homing within BM or liver or spleen and in the maintenance of the hemopoietic graft within these tissues, will be studied in SA#1 using genetic models with conditional VCAM-1 ablation. The contribution of Gi protein signaling pathway in homing and in the retention of cells within BM niches will be the focus of SA#2. Finally, in SA#3, we will extend our new observations that BM derived cells residing in muscle retain hemopoietic reconstituting activity and we will test the kinetics, the turnover post-transplantation, and their potential expansion through drug-induced (CID) proliferation. As BM-derived cells, both CD45+ or CD45-, have become an important paradigm of postnatal stem cell plasticity, these studies will provide a glimpse of a previously unappreciated tissue distribution and survival of BM derived cells, a new facet in BM physiology.